1. Technical Field
This application invention relates to torque transmission mechanisms that may produce a relatively large rotational torque. In particular, the present invention relates to torque transmission mechanisms that may be suitably utilized to power tools in order to rotate a tool spindle with a large rotational torque for tightening fasteners, e.g., screws, bolts and nuts. The present invention also relates to power tools incorporating such torque transmission mechanisms.
2. Description of the Related Art
FIG. 10 shows a known impact screwdriver 150 that has a rotary impact mechanism 160. The impact screwdriver 150 also includes an electric motor 151, a planetary gear mechanism 152 and a spindle 159 that is rotatably driven by the electric motor 151 via the planetary gear mechanism 152. The planetary gear mechanism 153 includes a sun gear (pinion gear) 157 that is mounted on an output shaft 151a of the electric motor 151. The rotary impact mechanism 160 is disposed on the front side (right side as viewed in FIG. 10) of the spindle 159.
The rotary impact mechanism 160 includes an anvil 161 and a hammer 162. The anvil 161 can rotate about the same axis as the rotational axis of the spindle 159. The hammer 162 has a substantially cylindrical tubular configuration and is fitted on the spindle 159, so that the hammer 162 can rotate and axially move relative to the spindle 159. The anvil 161 is rotatably supported by an impact casing 154 via a bearing 155. The impact casing 154 is attached to the front end of a main casing 153. The front end of the anvil 161 extends forwardly of the impact casing 154 and a driver bit (not shown) may be attached to the extended front end of the anvil 161.
Steel balls 164 are interposed between the spindle 159 and the hammer 162. More specifically, the steel balls 164 engaged respective cam recesses 159a that are formed in the outer peripheral surface of the spindle 159. The steel balls 164 also engage respective guide recesses 162a formed in the inner peripheral wall of the hammer 162. Each of the cam recesses 159a has a substantially semi-circular configuration in a cross-sectional and has a substantially V-shaped configuration as view from a lateral direction. The branches of the V-shape of each cam recess 159a are inclined relative to the rotational axis of the spindle 159. Each of the guide recesses 162a also has a substantially V-shaped configuration as viewed in a lateral direction but is oriented opposite to the cam recesses 159a. Therefore, the hammer 162 rotates relative to the spindle 159, while the hammer 162 moves in forward and rearward directions (right and left directions as viewed in FIG. 10) along the longitudinal axis of the spindle 159.
A compression spring 163 biases the hammer 162 in the forward direction (axial direction of the spindle 159), so that the movement of the hammer 162 in the rearward direction is performed against the biasing force of the compression spring 163. A pair of impact projections 162a are formed on the front end surface of the hammer 162 and extend toward the anvil 161. A pair of impact arms 161a extend from the rear end of the anvil 161 in a radial direction and serve to cooperate with the impact projections 162a. 
Because the hammer 162 rotates as the hammer 162 moves in the forward direction against the biasing force of the compression spring 163 as described above, the impact projections 162b of the hammer 162 may strike the impact arms 161a of the anvil 161. Therefore, the anvil 161 receives impacts in the rotational direction. As a result, the screws on be tighten by the driver bit that is mounted on the anvil 161.
When a load (tightening resistance) that exceeds a predetermined value is applied to the anvil 161 via the driver bit during the tightening operation, the hammer 162 rotates relative to the spindle 159 while the hammer 162 moves in the rearward direction, so that the impact projections 162b no longer strike the impact arms 161a. In other words, the hammer 162 is disengaged from the anvil 161 and the load is not applied to the hammer 162. Therefore, the hammer 162 rotates and moves in the forward direction by the biasing force of the compression spring 163. When the hammer 162 has rotated by an angle of about 180° after disengagement of the hammer 162 from the anvil 161, the impact projections 162b again strike the impact arms 161a, so that additional impacts are applied onto the anvil 161 in the rotational direction in order to further tighten the screws.
According to the known impact screwdriver 150, the impact projections 162b of the hammer 162 strike the impact arms 161a of the anvil 161 in the rotational direction in order to apply a large torque for tightening screws. Therefore, impact sounds are produced at each time when the impact projections 162b strike the impact arms 161a. Such impact sounds may cause a noise problem during the operation of the impact screwdriver 150.